Telephone system



6 Sheets-Sheet 1 Dec. 10, 1940- R. H. HERRICK TELEPHONE SYSTEM FiledFeb. 24, 1959 @u Po N@ @NQ w` Roswell H Herr/ck BY WZL fmZ/ ATTORNEYSDec. 10, 1940. R, H. HERRICK TELEPHONE SYSTEM Filed Feb. 24, 1939 6Sheets-Sheet 2 xbm@ NR .QQN ,wauw k, ..C w3@ m m Nm n @Nw Em SEQ w w#\I|. S M N l S om Smm |L 36 l SNE LNG 25 2 m @6mm R www1 h @C 3S @QQ SG$05 @QQ @www @www www@ @www @SQ w @NND ATTORNEYS Dec. 10, 1940. R. H.HERRlcK TELEPHONE SYSTEM 6 Sheets-Sheet 5 Filed Feb. 24, 1939 NNN,

Roswell fi Herr/'Ck ATTORNEYS Dec. 10, 1940. R. H. HERRICK TELEPHONESYSTEM Filed Feb. 24, 1959 6 Sheets-Sheet 4 INVENTOR Roswell H He BYiard/1?, f5/Lv@ rr/ck ATTORNEYS DeC 10, 1940- R. H. HERRICK 2.224,477

TELEPHONE SYSTEM Filed Feb. 24, 1939 6 Sheets-Sheet 5 INVENTOR Romae h.HQfr/'G/f BY im JLM@ ATTORNEYS Dec. `10, 1940. R. H. HERRlcK TELEPHONESYSTEM e sheets-shea e Filed Feb. 24,' 1939 @www m .mi

L @Q5 www@ 1N VENTOR. Rosale/ H Herr/Ck ATTGRNEYS QQ@ X S IQWV ,.IL @Enes@ @EQU [JIJbw du ,SS www# S $56 Patented Dec. 10, 1940 UNITED STATESPATENT vOFI-ICE TELEPHONE SYSTEM Application February 24, 1939, SerialNo. 258,264

30 Claims.

The present invention relates to improvements in telephone apparatus ofthe general character disclosed and claimed in applicants copendingapplication Serial No. 207,433, led May 12, 1938, now Patent No.2,187,902, granted Jan. 23, 1940. More particularly, thisy inventionrelates to improved telephone apparatus for providing simplified andmore direct calling facilities between the telephone substation of animportant executive or oicial and the telephone substations ofsubordinates who are frequently called from the executive telephonesubstation.

As explained in the above referred to patent, it sometimes occurs thatone party located at a given substation may have occasion to call thetelephones of a particular group of other parties at frequent intervals.This is particularly true of an executive who relies upon telephonefacilities to secure information from his immediate subordinates. In asituation of this character, the delay incident to the operation of amanual system or the labor incident to the operation of the callingdevice in automatic systems may prove burdensome to the party initiatingthe calls. In order to simplify the calling operation, the improveddirect call system disclosed in the above-mentioned copendingapplication was developed. This system is so arranged that calls to anumber of telephones may be initiated at the special substation eitherby using a conventional dial operated calling device or by the simpleexpedient of selectively actuating calling keys individuallycorresponding to the telephones which are frequently called. In thedisclosed arrangement, there is provided an automatic telephone systemincluding an exchange at which the plurality of lines of the systemterminate. VOne of the lines of the system extends to a special orexecutive substation of the character mentioned above. Automaticswitching equipment of the impulse responsive type is provided in theexchange for establishing connections between the lines extending to theexchange, the arrangement being such that the automatic switchingequipment is controllable from the executive substation or by way 0f anyof the lines extending to the exchange, to establish communicationconnections between the executive substation and the lines. There isIalso provided, in the exchange, special automatic switching meanshaving access only to the group of subordinate lines to which calls arefrequently initiated at the executive substation. This special automaticswitching means is individual to the executive substation and iscontrollable only from this substation to establish a secondcommunication connection between the substation and a second of thelines extending to the exchange. I'his second communication connectionmay be set up in the absence of a connection of the first type asmentioned above, or may be set up while a connection of the rst typeexists and without releasing the same. More specifically, there isprovided, at the executive substation, a plurality of key operatedcalling switches which individually correspond to the subordinate linesforming the group accessible to the automatic switching means men.-tioned above, and control apparatus operative in response to theoperation of any one of the calling switches for causing the specialswitching means to extend a connection from the executive substation tothe particular one of the subordinate lines corresponding to theoperated calling switch. For the purpose of communicating over aconnection of the rst type, there is provided, at the special orexecutive substation, the usual transmitter and receiver preferablyarranged in the form of the handset, which two elements are normallyconnected and arranged to be used in conversing over a connection of therst type. Auxiliary transmitting and receiving means are also providedat the special substation which are normally connected and arranged tobe used in communicating over a connection of the second type. Switchingapparatus is provided for operatively connecting the auxiliarytransmitting and receiving means to be used in communicating over eitherof the two types of connections briey described above. Moreparticularly, a pair of relays are provided which are under the controlof manually operable answer and transfer switches located at thesubstation and are operative selectively to connect the auxiliarytransmitting and receiving means to be used in communicating over anestablished connection of either type. The arrangement is such that thetwo relays are `iointly operative to transfer the auxiliary transmittingand receiving means from a connection of one type to a connection of theother type. 'I'he answer switch includes contacts for completing theoperating circuits of both relays, and the transfer switch includescontacts for interrupting the operating circuit of the transfer relaywithout deenergizing the other relay, thus providing an arrangementwherein the desired transfer of the auxiliary transmitting and receivingmeans may be effected by selective operation of the answer and transferswitches. In one embodiment of the apparatus, there is provided a relaywhich is operative in response to the removal of theV substationreceiver from its supporting hook or cradle to connect the handsetreceiver to be used in lieu of the auxiliary receiving meanscommunicating over a connection of the second type, namely, a directline connection. While perfectly satisfactory in operation, thearrangement is such that control of the lastmentioned switching relay isachieved only through modification of the subset to include an extra setof hook switch springs, thus rendering a standard substation instrumentunsuitable for use.

It is an object of the present invention to provide in a system of thecharacter described above an improved and exceedingly simple substationcircuit arrangement wherein either of two sets of transmitting andreceiving apparatus, one set of which comprises a standard subset, mayselectively be connected to an established communication connectioninvolving the substation.

It is a further object of the invention to provide in a system of thecharacter described an improved substation circuit arrangement whereineither of two sets of transmitting and receiving apparatus one set ofwhich comprises a standard telephone subset, may selectively beconnected to 'either of two established connections extending to thesubstation.

It is another object of the invention to provide in a system of thecharacter described, wherein two communication channels maysimultaneously be established between the executive substation and twodifferent lines of the system, an improved arrangement whereby thechannel which is not in use may effectively be short-circuited, therebyto prevent cross talk between the two channels.

It is a further object of the invention to provide an improvedarrangement controllable from the executive substation for marking theline over which incoming calls to the executive substation are routedlas busy in the numerical switches having access to this line withoutbusying any of the eoupment in the exchange.

'Ihe invention is illustrated as being incorporated in an automatictelephone system of the general arrangement outlined above. Inaccordance with one feature of the present invention, the arrangementprovided at the executive substation for selectively connecting theregular substation subset and the auxiliary transmitting and receivingmeans to an established connection involving the executive substationcomprises a first relay having a winding serially included in theregular substation line over which incoming calls are routed to thesubstation. and a second relay which functions to switch the connectionfrom one set of transmitting and receiving apparatus to the other. Thefirst relay is arranged to be energized when a loop circuit isestablished by way of the above-mentioned line. which occurs when thereceiver of the subset is removed from its supporting element. Theoperating circuit for the second relay is jointly controlled by the rstrelay, the special automatic switching equipment and the transferswitching apparatus, mentioned above. More particularly, with either thespecial switching equipment or the transfer switching apparatusoperated, the operating circuit for the second relay is completed inresponse to operation of the first relay. This second relay not onlyfunctions to perform the line switching operations noted,

but, in addition, functions to complete a holding circuit seriallyincluding its own winding and the winding of the first relay, andrenders the auxiliary receiving means inoperative by short-circuitingthe input channel thereto. The indicated holding circuit is interruptedto release the two relays and restore the normal circuit arrangementwhen the receiver of the subset is replaced upon its supporting element.

In accordance with another feature of the invention, the above-mentionedarrangement for rendering inactive the one of the two establishedcommunication channels extending to the executive substation which isnot in use, comprises apparatus controllable by the transfer means forshort-circuiting the inactive channel. More particularly, the transferrelay, which responds to operation of the transfer switch to select thedesired channel is provided with contacts for effectivelyshort-circuiting each of the two channels when not in use.

In accordance with a further feature of the invention, the apparatus forguarding the executive substation against incoming calls routed over theregular substation line comprises apparatus controllable from thesubstation for busying the line to the automatic switching equipmenthaving access thereto. More particularly, a path, jointly controlled bythe transfer and answer switches, is provided for impressing a linebusying potential upon the private conductor of the line, thereby toprevent seizure of the line by the automatic switches. By virtue of thisarrangement none of the automatic switching equipment is tied up or heldout of service as a result of busying of the line.

the particular arrangement of the apparatus whereby the above andadditional operating features are attained.

The novel features believed to be characteristic of the invention areset forth with particularity in the appended claims. The invention,Iboth as to its organization and method of operation, together withfurther objects and advantages thereof, will best be understood byreference to the specification, taken in connection with theaccompanying drawings, in which Figures 1 to 6, inclusive, takentogether, illustrate a telephone system having incorporated therein theabove-outlined features of the present invention, and Fig. 7 is adiagram illustrating the manner of assembling the other figures of thedrawings to form a unified system.

Referring now more particularly to Figs. 1 to 6 of the drawings, theequipment there illustrated is, with the exception of the substationapparatus shown in Fig. 1 and the substation apparatus 600 shown in Fig.6, included in an automatic exchange of the type frequently used toprovide telephone facilities between the offices of a largemanufacturing organization. This type of exchange is commonly known as aprivate automatic exchange. Extending to the exchange and terminating inautomatic switching equipment diagrammatically illustrated at 320 are aplurality of substation lines, three of which are indicated at 32|, 322and 323. The ends of the lines remote from the exchange terminate insubstations of the automatic type, that associated with the line 32|being diagrammatically illustrated at B and that associated with theline 322 being indicated A. The line 323 extends to a special orexecutive substation of the character mentioned above and includes twoline conductors C321 and C328 and a private or control conductor C323.The automatic equipment 320 comprises the usual subscriber-controlledautomatic switches for setting up connections between the telephonelines of the system. More particularly, these switches are, inaccordance with conventional practice, divided into two classes, namely,non-numerical switches, such, for example, as line finder or lineswitches, and numerical switches, such, for example, as selector andconnector switches, which latter switches are responsive to impulsestransmitted thereto over calling ones of the telephone lines andfunction to perform the usual line group, subgroup and unit selectingoperations. 'I'he connector switches at which the lines terminate are,in accordance with conventional practice, equipped to perform the usualauxiliary functions of testing a selected called line to determine theidle or busy condition thereof, returning busy tone over the callingline in the event a selected called line is found busy, and projectingringing current over a selected called line to actuate the signal deviceat the substation in the event the called line is idle at the time it istested. Further in accordance with conventional practice, the linesextending to the automatic switching equipment 320 are, when busy,guarded against seizure by the connector switches having access theretothrough the application of ground or line busying potential to thecontrol conductors thereof. For example, when the line 323 is busy.ground potential is impressed upon the control conductor C320 so thatnone of the connector switches having access to this line may seize theline.

The substation apparatus 600 and the apparatus provided at each of thesubstations A, B, etc., may be of any desired commercial constructionand arrangement. In brief, this apparatus may comprise the usual handsettype of transmitter and receiver, a cradle or hook comprising supportingmeans for the receiver, a cradle or hook switch, a ringer, and animpulsing device for transmitting switch control impulses by way of theassociated lines to the automatic switching equipment 320 located in theline terminatingr exchange. Preferably,y the elements of each substationcircuit are connected and arranged to form the well-known anti-side-tonecircuit, whereby operation of the substation transmitter does not causeoperation of the substation receiver.

-As pointed out above, the present invention relates to the provision ofimproved apparatus for increasing the utility of the available telephonefacilities at the executive substation where the substation apparatus600 is located. With the apparatus thus far described, communicationconnections may be established between the executive substation and anyof the other substa-V tions associated with lines terminating at theautomatic switching equipment 320 in the usual manner. Moreparticularly, on regular calls initiated at the executive substation,the calling device or dial operated impulsing device included in theapparatus 600 is actuated in the usual manner causing impulse trains tobe transmitted to the automatic switching equipment 320. An auxiliarydial operated calling device 350 may also be provided ii' desired, thisdevice being required for initiating calls under certain conditions ofoperation described in detail hereinafter. The automatic switchingequipment responds to impulse trains transmitted thereto by routing theconnection to the desired called line. All calls incoming to theexecutive substation are routed by way of the regular automaticswitching equipment 320 provided in the exchange and the line 323 to thesubstation apparatus 800, the signal device or ringer 64| embodied inthis apparatus being energized in response `to ringing current projectedover the line 323 following seizure of the line by one of the connectorswitches included in the automatic switching equipment 320.

For the purpose of routing calls from the executive substation directlyto certain of the frequently called subordinate lines terminating at theautomatic switching equipment 320 without operating a calling device ofthe type conventionally used in automatic systems and without the delayincident to the operations required to set up a connection in a manualsystem, there is provided automatic switching means comprising a rotaryswitch 3 I 0 having bank contacts at which the group of subordinatelines terminate. This switch is a standard twenty-five point rotaryswitch and comprises four sets of bank contacts 3|| to 3M, inclusive,wipers 3|5 to 3|B, inclusive, respectively associated with theenumerated contact sets, and means comprising a motor magnet 3I9 fordriving the enumerated wipers over the contacts oi' their respectiveassociated sets. w As indicated, the contacts in the two sets 3|3 and 3Mterminate the line conductors of the respective subordinate lines whichare directly accessible to the executive substation, and the contacts inthe set 3|2 terminate the private or control conductors of thesubordinate lines. The contacts of the fourth contact set 3| I terminateconductors extending by way of the cable 295 to the contact springs of agroup of coding relays shown in Fig. 2. These conductors individuallycorrespond to the subordinate lines and are included in marking circuitsby means of which operation of the switch 3|0 is controlled. Since thecontrol apparatus described hereinafter is only varranged to providedirect calling facilities to twenty subordinate lines only twenty of thecontacts of each contact set embodied in the switch 3|0 are used, theremaining contacts being left blank. switch 3|0 is arranged to becontrolled by the group of coding relays shown in Fig. 2 and by thecontrol Aapparatus 4|5 illustrated in Figs. 4 and 5. More particularly,the coding relays shown in Fig. 2 and indicated at R200, R2|0, R220,R230. R240, R250 and R260, respectively, are arranged to be energized incombinations of three in response to the selective operation of any oneof the plurality of calling switches illustrated in Fig. l. Theseswitches, which are indicated at |00 to |09, respectively, individuallycorrespond to the lines forming the group of subordinate lines directlyaccessible to the executive substation. Each of the enumerated callingswitchesis of the key operated type and is operable from a normalposition to either of two off-normal positions individuallycorresponding to two subordinate lines. Each of the switches is of thewell-known locking type, such that, when once operated to an olfnormalposition, it remains in its operated posi tion until manually restoredto normal. The conn the above-mentioned marking paths extending to oneof the contacts in the set 3H of the rotary switch 3|0, thereby toidentify the called line desired by the calling party.

The line wipers 3 I 1 and 3 8 of the rotary switch 3|0 are arranged tobe connected'by way of a link line comprising the conductors C335 andC339 to a repeating transformer 500 through operation of the controlapparatus indicated generally at 4|5. This apparatus comprises a startrelay R450 which is arranged to be energized in response to any one ofthe calling switches shown in Fig. 1 and functions to initiate theoperation of the rotary switch 3|0 to select a desired called line.Operation of the rotary switch is controlled by the three relays R420,R430 and R440, of which the relays R430 and R440 directly control thestepping operation of the switch 3|0 and the relay R420 is a test relayl or arresting the operation of the rotary switch when its wipers areoperated to a position corresponding to a desired called line. The relayequipment also includes a pair of relays R400 and R4I0 which function totest the condition of a selected called line to determine whether it isidle or busy. More particularly, the slow-torelease relay R4I0 is a busyrelay which is operative to return busy tone to the executive substationin the event a selected called line is found busy, and theslow-to-operate relay R400 is operative in the event a selected calledline is found idle and is arranged to control the ringing relay R560,shown in Fig. 5. The busy relay R4|0 also functions as a device forimpressing a background signal on an established connection involving abusy subordinate line called from the executive substation. Theapparatus 4|5 also includes a ring cutoi relay R550 and a back bridge orcalled party answer relay R5|0, which relays are arranged to perform thefunctions conventionally required of relays of this character.

As indicated above, auxiliary transmitting and receiving means areprovided at the executive substation for use in communicating overconnections established through operation of the auxiliary or specialswitching means just described. The auxiliary transmitting meanscomprises a microphone |35, and the auxiliary receiving means comprisesa sound reproducing device in the form of a loud speaker,diagrammatically illustrated at |36, connected in tandem with a signalamplifier including the vacuum tube 620. The auxiliary transmitting andreceiving means are interconnected with an anti-side-tone impedancenetwork indicated generally at 630 to form a substation circuit of thewell-known antiside-tone type having line terminals 643 and 644 arrangedto be connected by way of a link line including the conductors C515 andC516 to one winding of the repeating transformer 500 or by way of asecond link line comprising the conductors C296 and C291 to one windingof a second repeating transformer 300, shown in Fig. 3. Moreparticularly, the anti-side-tone impedance network 630 comprises aninduction coil or transformer 6I0 including five windings 6| I to 6|5,in-

clusive, the last-mentioned of which is coupled to the primary winding 6I6 of an input transformer 6|8 having its secondary winding 46|`|connected across the non-adjustable terminals of a voltage dividingresistor 6|9. The adjustable terminals of the resistor 6|9 are coupledto the control grid 62| and the cathode 622 of the tube 620. The tube620, which is of the well-known pentode type and includes a directlyheated cathode, is promary winding 628 of an output transformer 621connected in series with a source of anode potential between the anode623 and the cathode 622. For the purpose of biasing the control grid 62|negatively with respect to the cathode 622, there is provided a resistorG25 which is commonly included in the output circuit of the tube and theenergizing circuit for the 'cathode 622. The anode current source isby-passed for signal frequency currents by a condenser 626. Thesecondary winding 629 of the transformer 621 is connected to impress thesignal voltage developed thereacross upon the input terminals of theloud speaker |36 through a volumeA control potentiometer 4|. Thispotentiometer includes an adjustable contact carrying arm |45 soarranged that adjustment of the arm varies the magnitude of the signalvoltage impressed upon the input introducing distortion into thereproduced output of the loud speaker |36.

The anti-side-tone impedance network 630 includes, in addition to thetransformer 6|0 a pair of line balancing circuits which are arrangedselectively to be included in the network and are individuallyeffective, under different operating conditions, to balance the circuitso that, during operation of the microphone |35, no substantial portionof the developed signal voltage is impressed on the input circuit of theamplifier tube 620. The first of the two balancing networks comprises anadjustable inductance element 63| connected in series with an adjustableresistor 632, and the second network similarly comprises an adjustableinductance element 63| connected in series with an adjustable resistor632'. As shown; the transmitter microphone |35 is included in asubstantially closed transmission circuit comprising this microphone, alow impedance condenser |39, and the primary winding |38 of a couplingtransformer |31. .This transformer is provided with a secondary winding|40, one terminal of which is connected by way of the conductor C|46 tothe junction point between the two transformer windings 6II and 6|2 andthe other terminal of which is connected by way of the conductor C|41 tothe junction point between the two transformer windings 6|3 and 6|4. Thetwo junction points noted form the conjugate terminals of theanti-side-tone bridge circuit, which is effectively formed when theauxiliary substation equipment is connected for. use in communicatingover an established connection. More particularly, signal currentstraversing the secondary winding |40 of the transformer |31 duringoperation of the microphone |35 divide at the two junction points noted,portions of the currents traversing a path comprising the two windings6|| and 6|3 and one of the two balancing circuits, described above, andother portions of the currents' traversing the two windings 6|2 and 6|4and the established loop circuit extending to the repeating transformer500 or the repeating transformer 300. By suitably proportioning theimpedance values of the elements respectively included in the twobalancing circuits, the currents traversing the windings 6|3 and 6|4 arerendered substantially equal in magnitude and opposite in phase at allfrequencies within the audio range irrespective of which of the tworepeating transformers 300 and 500 is included in the establishedconnection. Similarly, when the impedance values of the elementsincluded in the balancing circuits are proportioned in the mannerindicated, the signal currents respectively traversing the two windings6|| and 6|2 are rendered substantially equal in magnitude and oppositein phase at all frequencies within the audio range. Thus, duringoperation of the transmitter microphone |36, the currents traversing thewindings 6H, 6|2, 6|3 and 6|4 cause substantially equal and oppositesignal voltages to be induced in the secondary winding 3|5, so thatsubstantially no signal energy is delivered to the input circuit of thevacuum tube 620. By virtue of this arrangement, suppression of the sidetone is achieved.

For the purpose of utilizing the auxiliary transmitting and receivingmeans to answer calls incoming to the executive substation and routed byway of the substation line 323, and for the further purpose ofpermitting the auxiliary transmitting and receiving means selectively tobe connected for use in communicating over either of two simultaneouslyestablished connections, each extending to the executive substation,there are provided an answer switch III, a transfer switch ||0, therepeating transformer 300, the link line comprising the two conductorsC296 and C231, and two control relays R210 and R520. 'I'he key operatedswitches and ||I are of the wellknown locking type. 'Ihe transfer switch||0 in combination with the start relay R450 also forms a part ofapparatus, controllable from the executive substation for marking theline 323 Aas busy in the bank contacts of the connector switches havingaccess thereto. Both of the two relays 40 R210 and R520 are arranged tobe energized in response to the operation of the answer switch III, thefirst thereof being operative to condition the signal amplier comprisingthe tube 620 for operation, and alsoto complete an auxiliary 45 loopcircuit extending to a distant subordinate substation and terminated atthe windings 303 and 304 of the repeating transformer 300. Arranged tobe serially included in an established auxiliary loop circuit is anadjustable resistor 306 50 for conducting direct current and by-passedfor signal frequency currents by a condenser 305. The relay R210conditions the signal amplifier for operation by supplying anode andcathode energizing voltages to the tube 620 over circuits tracedhereinafter. In this regard, it is pointed out that the tube 620 is ofthe well-known C33 type, which type of tube requires only a low anodevoltage to provide reasonably good amplification. Hence, the exchangebattery is used to supply 60 anode and screen voltages to this tube. Theexterminals 643 and 644, from a connection withr the winding50| of therepeating transformer 500 to a connection with the winding 30| of the 75repeating transformer 300. This relay also functions selectively toinclude the two balancing circuits, described previously, in theanti-slde-tone impedance network 630 and to short-circuit thecommunication channel not in use, all in the manner pointed out more indetail hereinafter. In order to remind a user of the' executivesubstation equipment that the answer switch I I is in its operated orofi-normal position, there is provided a signal lamp |26 which isarranged to be energized in response to the operation of the switch |I|to its off-normal position.

For the purpose of preventing an established connection extending to oneof the subordinate substations from being reestablished after the usualrelease operation is performed at the subordinate substation and beforethe. connection is released at the executive substation, there areincluded in the control apparatus 4i5 two relays R530 and R540, thefirst of which is of the slowto-operate type, and the last-mentioned ofwhich is of the slow-to-release type. More particularly, current forenergizing the transmitter microphone |35 is normally supplied to thismicrophone over a circuit including the winding of R540. When, however,a release operation is performed at a subordinate substation involved inan established connection extending to the executive substation, therelay R540 cooperates with the back bridge relay R5|0 to cause theoperation of the relay R530, and the last-mentioned relay functions toopen the circuit for supplying energizing current to the transmittermicrophone |35 and to maintain this circuit open so long as theconnection is not released at the executive substation.

For the purpose of rendering the inclusion of the regular substationtransmitter and receiver in an established direct call connection underthe control of the hook or cradle switch conventionally included in theregular substation appa.-V ratus 600, the three relays R610, R600 andR630 are provided. The arrangement is such that, when the transmitterand receiver hand set is being supported upon its hook or cradle, theloud speaker |36 and the microphone |35 are utilized as the transmittingand receiving means, respectively, and, when the substation hand set isremoved from its supporting hook or cradle, the transmitter and receiverembodied therein are substituted for the elements |36 and |35 as thereceiving and transmitting means to be used.

In order to provide secretary calling facilities there is also provideda call buzzer 642 which is arranged to be energized in response tooperation of a non-locking call key |42 over a circuit including oneconductor CMS of the two leads coupling the loud speaker |36 to theoutput transformer 621. It will be noted that a circuit including thewindings of the loud speaker |36, the conductor C|40 and the transformerwinding 626 is connected in parallel with the conductor C|49 comprisinga part of the energizing circuit for the buzzer 642. The impedance ofthis parallel circuit is, however, so high as compared with theimpedance of the conductor CI49, that it is traversed only a negligibleportion of the current for energizing the buzzer. Hence, operation ofthe buzzer does not cause any substantial reproduction of the pulsatingdirect current lby the loud speaker.

Although the apparatus shown in detail in Figs.

1 to 6, inclusive. may be arranged in any desired l manner, it iscontemplated that only the apparatus shown in Fig. 1 and the substationapparatus 600 shall be located in the office of the executive for whoseuse it is provided. It is further contemplated that the remainingapparatus illustrated in the drawings with the exception of the buzzer642 shall be conveniently located in the exchange terminating thevarious lines-of the system, a suitable multiconductor cable beingprovided for interconnecting the circuit elements to provide the circuitarrangement depicted in the drawings and described above.

Referring now more particularly to the operation of the specialswitching equipment to establish a connection to one of the regularautomatic substations and assuming that the substation A is beingcalled, the call is initiated by operating the calling switch |00 to theparticular oft-normal position corresponding to the called substation.More particularly, if the substation A is being called, the key switch|00 is operated from its normal position to its lower off-normalposition to close the springs of the three sets I I4, ||5 and I I6. Asindicated above, three of the coding relays illustrated in Fig. 2respond to the operation of any one of the key switches to an olfnormalposition to provide a marking path for marking the particular calledline in the contact level 3|| of the rotary switch 3|0. In the caseunder consideration, when the springs ||4, ||5 and ||6 are closed,circuits are completed for energizing the. three relays R200, R2|0 andR220, which circuits commonly include the winding of the start relayR450. 'I'he operating circuits for the three relays noted may beconsidered as a single circuit extending from battery at the lowi erterminal of the winding of R450 by way of this winding to C293, where itdivides, one branch extending by way of the winding of R220, C|32 andthe springs ||4 to the grounded conductor C|21; a second branchextending by way of R2I0, C|33 and the springs ||6 to the groundedconductor C|21; and thethird branch extending by way 0f R200, CI34 andthe springs ||5 to the grounded conductor C|21. When energized over theabove-traced circuits, the four relays R200, R2l0, R220 and R450 operateand the rst three thereof function to complete a marking pathidentifying the line 322 as the line being called. This marking pathextends from ground at RA20| by way of RA2| I, RA22| and C211 to thetwentyfifth contact of the contact set 3| I. Dueto the above-describedarrangement of the coding relays, none of the other marking pathsindividually corresponding to the other lines directly accessible to thespecial Vequipment are completed.

As a further example of the manner in which the coding relays operate inresponse to selective operation of the calling switches, it ispointed-out that, if the switch |00 is operated to close the contactsprings II2, ||3 and ||1, a series-parallel circuit is completed forenergizing the three coding relays R200, R2|0 and R230, and the startrelay R450. In this case, the two relays R200 and R2I0 are energizedover circuits substantially similar to the branch circuits traced abovey contact of the set 3| Again, 1f the tenth caning switch vme isoperated to its lower olf-normal position, the springs I2I, |22 and |23are closed to complete a seriesparallel circuit for energizing the threerelays R200.. R240 and R260 in series with the relay R450. In this casethe relay R200 is energized over a branch circuit extending by way ofC|34 and the springs |23 to the grounded conductor C|21; the relay R240is energized over a branch circuit extending by way of C|30 and thesprings |22 to the grounded conductor C|21; and the relay R260 isenergized over a branch circuit extending by way of C|28 and the springs|2| to the grounded conductor C|21. Upon operating, the three relayscomplete a path extending from ground at RA26| and by way of RA246,RA203 and C219 for impressing marking ground on the nineteenth contactof the contact set 3II. It will be apparent from the foregoingdescription that by selectively operating the calling switches, thecoding relays may be caused to operate in different combinations ofthree, thereby selectively to complete the twenty marking pathsextending to the respective contacts -of the conv tact set 3| I.

In any case when three of the coding relays are energized the startrelay R450 is also energized and, upon operating, completes, at RA452Vand RA453, respectively, paths commonly including the grounded armatureRA559 and C410 for applying ground to the tone apparatus and ringingmachine start conductors C401 and C408., thereby to initiate theoperation of the tone apparatus and the ringing apparatus in the usualmanner. AtvRA454, the relay R450 applies ground to the amplifier startconductor C294, thereby to condition the amplifier comprising the tube620 for operation. More particularly, operation of the relay R450results in the connection of the positive terminal of the exchangebattery to C294, so that anode potential is impressed through theprimary winding 628 of the output transformer 621 on the anode 623 ofthe vacuum tube 620. Operation of the relay R450 also results in thecompletion of a circuit for energizing the cathode 622 of the tube 620,this circuit extending from the grounded positive terminal of theexchange battery at RA454 by way of C294, the voltage dropping resistor624, the cathode 622 and the biasing resistor 625 to the negative.terminal of the exchange battery. It will be noted that positive screenpotential is applied to the screen electrode of the tube 620 directlyfrom the positive terminal of the exchange battery over the pathincluding the conductor C294. At RA455, the relay R450 prepares a path,traced hereinafter for applying ground potential to the privateconductor C329 of the line 323.

At RA45I, the relay R450 completes a circuit for energizing theslow-to-operate relay R430, this circuit extending from ground at RA45|by way of RA422, RC423, C336, the armature 3|9, C331 and the winding ofR430 to battery. The relay R430 operates, after an interval, and, atRA43|, prepares a circuit, traced hereinafter, for energizing the relayR420 when the wipers of the rotary switch 3| 0 are stepped to theposition corresponding to the selected called line 322. At RA432 andRC433, the relay R430 also prepares a test circuit, traced hereinafter,for

energizing the busy relay R4|0 in the event the F selected called lineis busy at the time the switch 3|0 is operated to the positioncorresponding to the called line. At RA435, the relay R430 completes acircuit for energizing the relay R440, this circuit extending fromground at RA45| by way of RA422, RC423, C336, the armature 3|0', C331,RA435 and the winding'of R440 to battery.

When energized over the circuit Vlust traced, the relay R440 operates tocomplete, at RA44I, a circuit for energizing the motor magnet 3|! of theswitch 3|0, this circuit extending from ground at the grounded contactRC421 by way of RA426, RA44I, C338, and the winding of the magnet 3I9 tobattery. Upon operating, the magnet 3|9 steps the wipers 3I4 to 3|0,inclusive, of the switch 3|0 one step and interrupts, at its associatedarmature 3I0', the above-traced operating circuits for the two relaysR430 and R440. The relay R440 immediately restores to interrupt, atRA44|, the operating circuit as traced above for the magnet 3 I 9,whereupon this magnet restores its associated armature 3|9 to recompletethe operating circuits for the two relays R430 and R440, respectively.When the operating circuit for R440 is again completed, this relayoperates to recomplete the above-traced circuit for energizing themagnet 3|9, causing this magnet to again step the enumerated Wipers ofthe switch 3|0 a second step and to again interrupt the operatingcircuits for the two relays R430 and R440. The above-describedinteraction between the relay'R440 and the magnet 3|9 continues untilsuch time as the wipers of the switch 3|0 are stepped to the positioncorresponding to the called line 322, or until the wiper 3|5 is movedinto engagement with the contact marked with ground over the abovetracedmarking path. During the interaction between'the relay R440 and themagnet 3|9, the relay R430, due to its slow-to-release characteristic,stays operated.

When the wiper 3|5 of the switch 3|0 is 0perated into engagement withits associated twenty-iifth contact, the above-mentioned circuit iscompleted for energizing the relay R420, this circuit extending from thegrounded conductor C211 by way of the twenty-fifth contact of the set3|I, the wiper 3|5, C333, RA43I and the lower winding of R420 tobattery. When its lower winding is energized over the circuit, justtraced, the relay R420 immediately operates to interrupt, at RA426 andRC421, the abovetraced circuit for energizing the magnet 3|9, thereby toarrest the interaction between this magnet and the relay R440 and toprevent further movement of the wipers embodied in the rotary switch3|0. At RA425, the relay R420 completes a holding circuit for itsself,this holding circuit extending from the grounded conductor C333, by wayof RA425 and the upper winding of R420 to battery. At RA422 and RC423,the relay R420 interrupts a point in 4 the circuits over which R430 andR440 are being held energized, causing these two relays to restore. AtRA422 and RC424, the relay R420 prepares a holding circuit, tracedhereinafter, for R560.

At RA42 I, the relay R420 completes the abovementioned circuit fortesting the called line 322 to determine the idle or busy conditionthereof. In this regard and as previously explained, the automaticswitching equipment provided in the exchange 320 is conventionallyarranged so that, when the line 322 is busy, ground potential is presentupon the control conductor C324 thereof, whereas, if the line is idle,battery potential appears on this control conductor. Assuming theselected called line 322 is busy, the busy relay R4I0 is energized overthe above-mentioned test circuit, which circuit extends by Way of thecontrol conductor C324, the twenty-fifth contact of the set 3|2. thewiper 3|6, C334, RA42I, RA432, RC433 and the winding of- R4|0 tobattery. Upon operating, the busy relay R4|0 interrupts, at RA4I I, apoint in the circuit, traced hereinafter, for energizing theslow-to-operate control relay R400. At RA4I2, the relay R4I0 completes aholding circuit for itself, this circuit extending by way of theconductor C334, RA42 I, RA4|2 and the winding of R4I0 to battery.

The relay R4|0, upon operating, functions to complete a circuit forimpressing busy tone voltage on the input circuit of the signalamplifier comprising the vacuum tube 620., Specifically, the path of thebusy tone current extends from the ungrounded terminal of the busy tonesource, not shown, by way of the busy tone conductor C406,the condenser404, RA4I3, C465, RC529, RA520, C516, RA616, the transformer windings6|2 and 6I I, the inductance element 63|, the resistor 632, C512, RC522,RA52I, C514, the transformer windings 3I3 and 6|4. RA61I, C515, RA524,RC525 and the upper half of the Winding 50| to the grounded terminal ofthe busy tone source at the grounded center tap 502 of the winding 50|.This current causes an induced voltage to be developed in thetransformer winding 6|5 which is impressed through the couplingtransformer 6I8 between the input electrodes of the tube 620, and theamplified output energyvis delivered through the transformer 621 and thevolume control potentiometer |4| to the loud speaker |36 forreproduction. The reproduced busy tone signal indicates to the callingparty that the desired connection cannot immediately be established.

Presumably, the substation at which the illustrated special equipment islocated is that of an important ollicial or executive, such that callsoriginating at this substation and' intended for the various subordinatelines are of suflicient importance to warrant direct attention on thepart of the called parties. For this reason, the busy relay R4| 0 is soarranged that it functions to impress a background signal on a selectedsubordinate line which is tested and found busy. More particularly, whenthe relay R4|0 operates, it completes a path for impressing the voltageof the ringing current source upon the previously established loopcircuit of which the called linc forms a part. In the case underconsideration, the path of the ringing current extends from theungrounded terminal of the ringing current source, not shown, by Way ofthe ringing current conductor C405, the condenser 402, RA4I4, C339, thewiper 3I6, the twenty-fifth contact of the set 3|4, the line conductorC326, the receiver at the called substation A, the line conductor C325,and

through one Winding of the back bridge relay conventionally provided inthe operated connector switch of the switch train leading to the line322, to the grounded terminal of the ringing current source. Thecondenser 402 included in this path is selected of a value such that thecurrent traversing the above-traced path is insuflicient to produce asignal `which will interfere with the ensuing conversation over theestablished connectionincluding the line 322, but is of suflicientstrength to indicate that the substation A is being called from theexecutive substation.

Following the operation of the busy relay R4I 0, no further operation'of the apparatus occurs until the partially established connection isre- 40 terrupted causing this relay to restore.

leased. To release the connection it is only necessaryto restore theoperated key switch to its normal position, thereby to interrupt theabove-traced operating circuits for the four relays R200, R2|0, R220 andR450, causing all of these relays to restore. When the first three ofthe enumerated four relays restore, the above- .traced marking pathextending to the twentyfifth contact of the contact set 3H in the switch3|0 is interrupted. When this path is interrupted, the holding circuitfor R420 is broken, causing this relay to restore to interrupt, atRA42I, a point in the above-traced holding cir:- cuit for the busy relayR4I0. Thus, the three operated relays R4|0, R420 and R450 are caused torestore and the common apparatus 4|5 is conditioned for further use. Inthis regard, it will be noted that when the relay R450 restores, itinterrupts, at RA454, the path for applying ground to the amplier startconductor C294, whereby the signal amplier including the tube 620 isrendered inactive. Upon restoring, the relay R450 also interrupts, atRA45I, a point .in the operating circuits for R430 and R440 and, atRA452 and RA453, the two paths described above for applying ground tothe tone apparatus start conductor C401 and the ringing apparatus startconductor C408.

If the calling party desires he may disregard the busy signal and waituntil the established connection is cleared out, in which case theapparatus 4|5 operates to extend the connection to the called line 322immediately the calledl line becomes idle. In this regard it is pointedout that the above-traced holding cli/cuit for the busy relay R4|0extends to grou `cl `as applied to the control conductor.C324. Hence,when the line 322 becomes idle and ground is removed from C324, the`holding circuit for R4H) is in- Upon . restoring, the busy relay R4|0permits ringing current to be projected over the called line in themanner described immediately hereinafter.

In the event the called line 322 is idle at the 45 time it is selectedby the switch 3|0, battery potential is present on the control conductorC324 thereof and, hence, the busy relay R4|0 is not energized. A shorttime interval after the switch 3|0 is operatedto the positioncorresponding to the line 322 or, more particularly, a short timeinterval after the relay R420 operates, the slow- 'to-release relay R430restores to interrupt, at RA432 and RC433, a point in the previouslytraced testing circuit by way of which the busy relay R4|0 is energized.At RA432 and RC434, the relay R430, upon restoring, completes a circuitfor energizing the control relay R400, this circuit extending fromground at RA45| by way of RA4||, the winding of R400, RC434, RA432,

60 RA42I, C334, the wiper 336 and its associated twenty-fifth contact tobattery as impressed upon the control conductor C324 of the idle line322. Shortly following its energization over the cir.- cuit just traced,the slow-to-operate relay R400 operates to complete, Iat RA40I, acircuit for energizing the ringing relay R560, this circuit extendingfrom ground at RA45I by way of RA422, RC424, RA40|, C463 and the windingof R560 to battery. Upon operating, the relay R550 completes, at RA561,a holding circuit for itself, which circuit exten-ds from ground atRA45| by way of RA422, RC424, C462, RA561 and the Winding of R560 tobattery. At RA568, the relay R560, upon operating, applies ground to thecontrol conductor C324 of the called line 3.22, thereby to mark .thisline as busy in the bank contacts of the connector switches to which itextends. Specifically, the above-mentioned marking path extends fromground at RA568 by way of C464, RA42I, C334, the wiper 3I6 and itsassociated twenty-fifth contact to the control conductor C324. At RA565and its associated grounded contact RC566, the re1ayfR560 prepares lalocking circuit, traced hereinafter, for thel slow-acting ring cut-olfrelay R550. At RA56| and RA562, therelay R560 completes a path forprojecting ringing current over the called line 322, thereby to energizethe signal device at this substation. Specifically, the path traversedby the ringing current extends from one terminal of the ringing currentsource, not shown, by way of C405, C461, RA551, RA56I, C330, the wiper3|3 and its associated twentyfifth contact, the line conductor C326, thesignal' device at the ycalled substation, the line conductor C325, thewiper 3|1, C335, RA563, RA562, RA552, RC553 and the upper Winding ofR550 to the other terminal of the ringing current source. The resultingoperation of the signal device provided at the called substationindicates that this substation is being called. The relay R560 alsocompletes a path for transmitting ring-back-tone energy to the inputcircuit of the signal amplifier tube 620, this path being substantiallythe same as that traced previously for -the busy-tone current, butextending from the ringing current source by way of C405, the condenser403, C468, RA569, RA558 and RA542, rather than by way of C406, thecondenser 404, RA4|3 and C465. 'I'he ring-backtone energy delivered tothe input circuit of the amplier tube 620 is ampliiied by this tube andv delivered to the loud speaker |36 for reproduction, thereby toindicate to the calling party that the desired called substation isbeing rung.

'Ihe desired conversational connection is completed when the call isanswered at the called substation `A. More particularly, when thereceiver at the substation A is removed from its hook, a direct currentbridge is placed across the line conductors C325 and C326 of the line322 to complete a direct current circuit substantially similar to that,-traced previously and traversed by the alternating ringing current, forenergizing the upper winding of the ring cutoff relay R550. When itsupper winding is energized the relay R550 iirst closes the X contactsassociated with RA556, thereby to complete the above-mentioned holdingcircuit for` itself, this circuit extending from ground at RC566 by wayof RA565 and the two windings of R550 in series to battery. At RA552 andRA551 and their respective associated resting contacts, the relay R550opens two points in the above-traced path for projecting ringing currentover the selected called line 322 and, in so doing, opens two points vinits own operating circuit. At these same arpletes, at RA53I, a holdingcircuit for itself, thisy phone |35 is conducted to the microphone overthe circuit Just traced. Upon` operating, the relay R540 prepares, atRA54I, a circuit, traced hereinafter, for energizing the relay R530. AtRA542, the relay R540 opens a further point in a path for conductingring-back-tone current by way of the anti-side-tone impedance network630 to the input circuit of the amplifier tube 620.

Following the operation of R540, the desired conversational circuit iscompletely established, talking battery being supplied to the loopextending to the called substation A through the windings of the backbridge relay R5|0, and talking battery being supplied to the transmittermicrophone |35 over the circuit, traced above, for energizing the relayR540. Signal currents incoming to the special substation and traversingthe windings 503 and 504 of the repeating transformer 500 cause inducedvoltages to be developed in the winding 50| coupled to the two windings503 and 506, whereby signal currents corresponding to the incomingsign-al currents are caused to iiowthrough the anti-side-tone impedancenetwork 630. More particularly, the path followed by currents traversingthe winding 50| may be traced as extending from the lower terminal ofthis winding by way of RC529, RA528, C516, RA616, the two windings 6|2and 6| I, the inductance element 63|, the resistor 632, C512, RC522,RA52I, C514, the two windings 5|3 and 6H, RAB'H, C515, RA524, RC525,`

back to the upper terminal of the winding 50|. The resulting inducedvoltages developed in the winding SI5 are impressed through the couplingtransformer 6|8 and the vol-tage dividing resistor 6|9 between the inputelectrodes of the amplier tube 620, thereby to cause correspondingamplified currents to flow in the output circuits of this tube. Suchcurrents cause induced voltages to be produced in the winding 629 of theoutput coupling transformer 621, which voltages are impressed throughthe potentiometer |4| on the input terminals of the loud speaker |36 forreproduction.

Audio frequency currents developed by operation of the microphone |35are caused to circulate in the closed low impedance transmission circuitcomprising the primary winding |38 of the transformer |31 and the'condenser |39. Such currents cause corresponding induced voltages to bedeveloped in the secondary winding |40 of the transformer |31 which areimpressed by means of the two conductors C|46 and C|4`| between oppositeterminals of the anti-side-tone impedance network 630. Moreparticularly, signal currents traversing the winding |40 divide at theterminals of the anti-side-tone impedance network and follow the twopaths traced previously, whereby the well-known anti-side-tone effect isrealized.

The release of the established communication connection is primarlyunder the control of the calling party. If the called party at thesubstation A restores his receiver to its hook before the calling keyswitch is returned to normal, the established called partly loop circuitextending to the repeating transformer 500 is interrupted, causing theback bridge relay R|0 to restore in the usual manner. Upon restoring,the relay R|50 interrupts, at RA5|| and RC5|3, the above-traced circuitfor energizing R540 and, at RA5|| and RC5|2, it completes a circuit forenergizing R530, the latter circuit extending by way of RA521, RA54| andthe winding of R530 to battery. Upon operating, the relay R530comcircuit extending from ground at RA555 by way of RA53| and thewinding of R530 to battery. It is noted that the relay R530, although ofthe slowto-operate type, operates to complete its holding circuit beforethe slow-to-release relay R540 restores to open the operating circuitfor R530. At RA532, the relay R530, upon operating, opens a furtherpoint in the above-traced operating circuit for R540 and, in so doing,opens a i'urther pnint in the circuit by way of which energizing currentis delivered to the microphone |35. At RA534, the relay R530 appliesground by way of C410 to the two start conductors C401 and C408, therebyto reinitiate the operation of the tone apparatus and the ringingapparatus providing this apparatus is not already in operation. At1`\.A542,l the relay R540. upon restoring, completes an alternative pathincluding C469 for impressing busy-tone voltage upon the input circuitof the signal amplifier tube 620, thereby to cause a busy-tone signal tobe reproduced by the loud speaker |36. The busy-tone signal produced byoperation of the loud speaker |36 indicates to the calling party thatthe key switch v |00 has not been restored to normal.

The purpose of providing the relay R530, which operates in response tothe release operation performed at the called substation, is that ofpositively preventing eaves dropping on the part of a subordinate,following the termination of a conversation between the subordinate andth calling executive. relay and its associated circuits, the calledparty at the substation A could, after hanging up, again remove hisreceiver from its hook to recomplete the talking connection, providedthe operated key switch |00 was not restored to normal bythe callingparty. In this regard, it will be appreciated that the back bridge relayR5|0 would respond to the recompletion'of the loop circuit to againcomplete the circuit for delivering energizing current to thetransmitter |35. Obviously, with this connection reestablished, thesubordinate could listen to any conversation being carried on in theoflice of the executive. With the arrangement illustrated, however, therelay R530, upon operating, is held operated independently of the backbridge relay R5|0 and, in its operated position, holds interrupted, atRA532, the above-traced path by way of which energizing current isdelivered to the transmitter microphone |35. Thus, the channel fortransmitting signal currents outgoing from the special substation to thecalled substation is rendered completely inactive and eavesdropping onthe Vpart of a subordinate is positively prevented.

As indicated above, the common control apparatus 4|5 is released inresponse to the restoration of the operated key switch 00 to its normalposition. When this switch is returned to normal the three operatedcoding relays R200, R2|0 and R220, and the start relay R450 are allcaused to restore in the manner pointed out above. When the threeenumerated coding relays restore, the above-traced holding circuit forR420 is interrupted, causing the last-named relay to restore andinterrupt, at RA422 and RC424, the circuit over which R560 is being heldenergized. During the restoration of the relay R560, the contact springsRA563 and RC564 are moved into engagement before the loop `circuitextending to the called substation is interrupted, provicling the calledparty has not restored his receiver to its hook. When the enumeratedsprings Thus, in the absence of this.

' ductors C339 and C335.' -This condenser absorbs the voltage transientwhich occurs when the talking-loopi is interrupted at RAEBI :followingthe restoration oi.' R560 completely to normal and thus prevents thetransient from being reproduced as an objectionable "click by thereceiver at the called substation. When R560 falls back, it interrupts,at RA565 and RC566, a point in theholding circuit for the ring cutoirelay R550, whereby the last-mentioned relay is caused to restore. Therelay R550,

upon restoring, opens at RA55| a point in the path connecting thecondenser 543 between the link line conductors C335 and C339. Uponrestoring, the relay R550 opens, at RA555, a point in the holdingcircuit for R530. If R530 is operated, lt restores to remove, at RA534,multiple ground from C410. Following the restoration of R530, the commoncontrol apparatus 4|5 is entirely released.

If the executive desires not to be disturbed by incoming calls routedover the line 323 while the above-described direct line call is inprogress, he may preclude such incoming calls by operating the transferswitch H0, thereby to mark the line 323 as busy in the bank contacts ofthe connector switches having access thereto. More particularly, whenthe start relay R450 operates it prepares a path for impressing groundpotential upon the private or control conductor C329 of the line 323.This path may be completed by operating the transfer switch H withoutoperating the answer switch lil and extends from ground by way of themiddle and left-springs of the. switch ill, the middle and left springsof the switch H0, CI52, RA213, 025| and RA455 to the private vconductorC329. When this path is completed,

the line 323 tests busy to all automatic switches attempting to gainaccess thereto and thus the executive substation is guarded against:incoming calls over this line. f

Following the completion of the direct line connectionin the manner justdescribed, the substation apparatus 600 may, if desired, be used in lieuof the auxiliary transmitting and receiving the apparatus 600 is restingupon its supporting cradle or hook, the loop circuit extending from thesubstation apparatus 600 by way of the line 323 to theswitchingapparatus 320 is open andthe relays R610, R660 and R690 aredeenergized, whereby the line conductors C515 and C516 are connected byway of the armatures RA61I and RA616 and their respective associatedresting contacts through to the lineterminals 643 and 644 of the network630. Also, with the hand set resting on its supporting means such thatthe relay R610 is restored, the two line conductors C321 and C328 of thesubstation line 323 are connected by way of the armatures RA614 andRA615 and their respective associated resting` contacts to the lineconductors of the cord extending to the substation apparatus 600. Solongas all of the calling switches and the answer key switch III occupytheir respective normal positions, the amplifier start conductor C294 isdisconnected from ground so that the operating circuit, tracedhereinafter, for R610 is interrupted independently of the hook or cradleswitch provided in the apparatus 600- may be utilized in the usualmanner in the initiation and answering of calls outgoing from andincoming to .the executive substation.

If, after the above-described direct call connection is established, itis desired to use the hand set embodied in the apparatus 600 as .thetransmitting and receiving means in communicating over the direct callconnection, the hand set is removed from its supporting hook or cradle,thereby to connect a direct current bridge between the line conductorsof the cord extending to the apparatus 600. As a result, a loop circuitis completed over which the relay R680 is energized, this circuitextending from one side of the exchange battery through one winding ofthe line relay associated with the line 323 and by Way of C321, RA614,the winding of R690, the bridge across the line conductors of the cordextending to the apparatus 600, RA615, C328, and through the otherwinding of the line relay associated with the line 323 to the otherterminal of the exchange battery. When energized over this circuit, therelay R680 operates to complete, at

' RA68| an obvious circuit for energizing the relay R690. Theslow-to-operate relay R690 now partially operates to close the "Xcontacts associated with RA693, thereby to complete a circuit forenergizing the relay R610, this circuit extending-by way of the groundedstart conductor C294,

1 RA693, RA692, RA613, and the lower winding of R610 to battery. Shortlythereafter, the relay R690 completes its operation to open, at RA692, a

point in the circuit just traced and to prepare, at

RA692, an alternative circuit, traced hereinafter, for energizing thelower winding of R610. At RA69I, the relay R690 interrupts the pathnormally short-circuiting its upper winding, thereby to render itselffast to release. The armature springs of the relay R610 are so adjustedthat the two armatures RA614 and RA615 are moved from engagement withtheir respective associated resting contacts and into engagement withtheir respective associated working contacts before the armature RA692of the relay R690 is moved out of engagement with its associated restingcontact. Thus, upon the initial energization ofthe lower winding ofR610, a holding circuit is completed for this relay and the relay R680before the operating circuit, just traced, is interrupted. I'his holdingcircuit extends from the grounded conductor C294 by .way of the upperwinding of R610, RA615 and its associated working contact, the closedbridge between the line conductors of the cord extending to theapparatus 600, the winding of R680, RA614 and its associated workingcontact, and the lower winding of R610 to battery. Energizing current isdelivered to the transmitter embodied in the hand set of the apparatus600 over this holding circuit. Immediately after the completion of theabove-traced holding circuit, the armature RA692 of the relayR690 ismoved from engagement with its associated resting contact to interruptthe operating circuit for R610.

At RA61I and RA616 and their respective associated resting contacts, therelay R610, upon fully operating, disconnects the line conductors C515and C516 from the line terminals 643 and 644, respectively, of theanti-side-tone impedance network 630 and connects these line conductorsby way of the condensers 618, 619 and 682 to the line conductors of thecord extending to the substation apparatus 600. Thus, the transmitterand receiver incorporated in the substation apparatus 600 aresubstituted for the auxiliary transmitting and receiving means in theestablished connection.

When the operating circuit for R610 is completed, a relatively heavycurrent iiows through the upper winding oi the relay, which current ismaterially reduced when the operating circuit is broken and the holdingcircuit for R610 is completed. Due to the collapse of the magnetic fieldoccasioned by interruption of the operating circuit, a relatively largetransient voltage tends to be developed across the windings oi.' R610,which, if not absorbed, would be impressed through the condensers 616and 619 across the repeating transformer winding 50| or the winding 30|,depending upon the position of the relay R520. If allowed to assume itsfull value, such a transient would cause an objectionable click in thereceiver being used at the distant end of the connection. To obviatethis dimculty, the relay R610 is provided with a set of armature springsassociated with RA611 which are included in an obvious path forshort-circuiting the signal channel comprising the two line conductorsC515 and C516 during the initial operation of R610. The springs of thisset are so arranged that they are not moved to the open-circuit positionto interrupt the short-circuiting path noted until after the transientis substantially damped out. By this arrangement the transient voltageis not4 impressed on the signal channel of which the line conductorsC515 and C516 form a part, but is dissipated in a closed circuitcomprising the condensers 610 and 619, the two windings of R610, thewinding of R660 as shunted by the by-pass condenser 602, the exchangebattery and RA611.

It will be noted that, when the relay R610 operates to connect thesubstation apparatus 600 to be used in communicating over theestablished direct line connection, the line terminals 643 and 40 644 ofthe anti-side-tone impedance network 630 are open-circuited and, thus,this network is unbalanced so that electrical coupling is incidentallyprovided between the signal current receiving and signal currenttransmission paths extending to the loud speaker |36 and from themicrophone |35, respectively. Also, acoustical coupling exists betweenthe microphone I 35 and the loud speaker |36, and the signal amplifiercomprising the vacuum tube 620 is still energized and in conditionfor-operation. Due to the two couplings noted, a local oscillatorycondition tends to be set up in the network comprising the signalamplier, the transmitter 35, the loud speaker |36, and theanti-side-tone impedance network. If this oscillatory condition isallowed to persist, howling or singing results. To obviate thisdifculty, the relay R610 is provided with a pair of' armature springsfor short-circuiting the input circuit to the loud speaker |36 when thisrelay occupies its operated position. More particularly, when the relayR610 operates, it completes, at RA612, an obvious path forshort-circuiting the winding 629 of the transformer 621. therevbyeffectively to interrupt the local oscillatory of the cord extending tothis apparatus. When this occurs, the above-traced holding circuit forthe relays R616 and R660 isinterrupted. Thus, the relays R610 and R660are caused to restore. Upon restoring, the relay R660 opens theoperating circuit for R690, causing the last-mentioned relay to restore.The relay R610, upon restoring, disconnects the substation apparatus 600from the established communication connection; interrupts, at RA612, theshort-circuiting path across the winding 629; and reconnects the lineconductors C515 andC516 to the line terminals 643 and 644, respectively,of the anti-side-tone impedance network 630.

From the foregoing description, it will be apparent that, when once therelay R610 is operated, it is only deenergized in response to thereplacement of the substation hand set upon its supporting hook orcradle. Hence, to release a direct call connection which has beenestablished in the manner previously described and to which theapparatus 600 isv operatively connected, it is necessary to restore theoperated one of the calling switches of Fig. 1 to normal and to replacethe substation hand set upon its supporting hook or cradle. When theoperated calling switch is returned to its normal position, the operatedrelays of Figs. 2, 4 and 5 are caused to restore in the manner describedpreviously, it being noted that, when the start relay R450 falls back,ground potential is removed from the ampliiier start conductor C294 toopen a point in the operating circuit for the relay R610. When thesubstation hand set is placed upon its supporting hook or cradle, theoperating and holding circuits for the relays R610, R660 and R690 areinterrupted in the manner just described.

As indicated above, the substation apparatus 600 is normally connectedand arranged to be used in communicating over connectionsestablishedbetween the executive substation and one of the other substations oncalls incoming to the executive substation. The special substationapparatus illustrated in Figs. 1 to 6, inclusive, is, however, soarranged that the auxiliary transmitting and receiving means comprisingthe microphone |35 and the loud speaker |36 may, if desired, be used inlieu of the regular substation apparatus 600 toanswer incoming calls.For example, if a call is initiated at the substation A and routed byway of the automatic switching equipment 320 and the line 323 to thesubstation apparatus 600 at a time when the line 323 is idle, ringingcurrent is projected over the line to actuate the signal device 64I,thereby to give an audible indication that the executive substation isbeing called. Obviously, the call may be answered in the usual manner byremoving the receiver embodied in the substation apparatus 600 fromlitsassociated hook. In this case the resulting senu'ential operation of thetwo relays R660 and R690 is without effect since ground potential is notpresent upon either of the conductors C294 and C694 and. hence, therelay R610 is not energized. Alternatively, the auxiliary transmittingand receiving means comprising the loud speaker |36 and the transmittermicrophone 35 may be used. In case the answering party elects to use thelast-mentioned means, the answer switch is operated to its off-normalposition to close the springs |25 and thereby complete a circuit forenergizing the control relay R210. This circuit extends from ground byway o1' the switch springs |25, the signal lamp |26, CI21 and thewinding of R210 to battery. Upon operating, the slow-to-releaserelay.R210 completes a lcircuit for energizing the relay R520. Thelastmentioned circuit extends from ground by way cf the springs |25, thesprings |24, CI52, RA213, and the Winding `of R520 to battery. Whenenergized over the above-traced circuit, the signal lamp |26 isilluminated to indicate that the answer switch ||I occupies itsoff-normal position. 'The relay R210, upon operating. also completes, atRA212, a path for applying ground to the amplifier start conductor C294,whereby the signal amplifier comprising the vacuum tube 620 isconditioned for operation in the manner described previously. At `RA212and RA214, the relay R210 prepares a circuit, traced hereinafter, forenergizing the relay R610. At RA21I, the relay R210 completes a directcurrent bridging path between the line lconductors C321 and C328 of thecalled line 323,

which path includes the two windings 303 and 304 of the repeatingtransformer 300. More particularly, the bridging path noted extends fromthe line conductor C321 by way of the winding 304, the resistor 306,C299, RA21I, C298 and the winding 303 to the opposite line conductorC328. The operated connector switch by way of which the connection isrouted to the line 323 responds to the completion of this path in theusual manner by interrupting the ringing operation.

As indicated above, the relay R520 is energized in response to theoperation of the answer switch lll to its off-normal position and theresulting operation of the relay R210. When energized, this relayoperates to complete, at RA528', the above-traced circuit for energizingR540 and by way of which energizing current is delivered to thetransmitter microphone |35. At RA521, the relay R520 opens a point inthe above-traced operating circuit for R530, thereby to prevent thelast-mentioned relay from operating in response to the operation ofR540. At RA523, the relay R520 opens a closed and obvious path normallyshort-circuiting the winding 30| of the transformer 300. At RA525, therelay R520 completes an obvious path for short-circuiting the winding50| of the transformer 500. The purpose of this channel short-circuitingarrangement is to preclude the possibility of cross talk. To this end,the relay R520 is provided with the'two armatures RA523' and RA525',which function to shortcircuit the channel not in use in an obviousmanner. associated resting contacts RC525 andi RC529', the relay R520disconnects the line conductors C515 and C516 from the winding 50| oftherepeating coil 500, and, at these same armatures and their respectiveassociated working contacts RC526 and RC529, connects the lineconductors C515 and C516 by wayof the link line conductors C296 andC201, respectively, to the opposite terminals of the repeatingtransformer Winding 30|.-

Thus, the repeating transformer 300 is incorporated in the newlyestablished talking connection in lieu of the repeating transformer 500,which latter transformer is used on subordinate calls originated at thespecial substation. Since the impedance characteristics of theloop'circuits respectively including the repeating transformers 500 and300 may be different, it is desirable to provide alternative linebalancing circuits arranged alternatively to be included in the. anti/-slde-toneimpedance network 630. As described above, with the relay R520deenergized, thel line balancing circuit comprising the inductanceelement 63| and the resistor 632 is operatively included in theanti-side-tone impedance network At RA524 and RA528 and their respective630. When, however, the relay R520 operates, it substitutes the otherAbalancing circuit, namely, that comprising the inductance element 63|and the resistor 632', for the first-mentioned balancing circuit in theanti-side-tone impedance network. This is accomplished through theoperation of R520 to disconnect, at RA52I, the conductor C514, extendingto the upper terminal of the winding 6|3, from the conductor C512 and toconnect this conductor to the conductor` C513. It will be understoodthat the line balancing circuit comprising the inductance element 63|'and the resistor 632' is adjusted to have a frequencyimpedancecharacteristic substantially matching that of the equivalent impedanceof the repeating transformer 300 and the line impedancesasso ciatedtherewith as determined at the line terminals 643 and 644 of theimpedance network 630. Similarly, the line balancing circuit comprisingthe inductance element 63| and the resistor 632 is adjusted to have afrequency-impedance characteristic substantially the same as that of theequivalent impedance of the repeating transformer 500 and the lineimpedances associated therewith as determined at the line terminals 643and 644 of the network 630.

Following the operation of R520, the desired conversational connectionis fully completed.

After the connection is completed the answering party may, by removingthe telephone receiver or hand set of the substation apparatus 600 fromits supporting hook or cradle, release the auxiliary transmitting andreceiving means comprising the loud speakerjl36 and the transmitter |35and continue the conversation by using the regular substationtransmitter and receiver. the receiver of the telephone instrument 600is removed from its supporting element the three relays R610, R680 andR690 respond in the exact manner previously described to disconnect thetelephone instrumentA of the apparatus 600 from the line 323 and toconnect this instrument to the link line conductors C515 and C516. Itwill be noted that with the relays R210, R520 and R610 oper-ated, thesubstation apparatus 600 is operatively connected to the line 323 overan alternative channel, namely, that including thelink line C515, C516,the link line C296, C291, and the transformer 300. With the relays R210and R520 operated it is obvious that, by restoring and removing thesubstation hand set from its supporting hook, the answering party maysubstitute one set of apparatus for the other as many times as desiredduring a single conversation. When, however, the answer switch isrestored to normal, the above-traced operating circuit for the relayR520 is interrupted .to cause this relay and the relay R540 to restorein the When order named, and the series circuit for energizing thesignal lamp |26 and the relay R210 is inter-` rupted causing thelast-mentioned relay to restore. When R210 restores, it removes groundfrom the amplifier start conductor C294, thereby to render the signalamplifier inactive and to open a point in the operating circuit forR610, and opens, at RA21I, a point in the above-traced bridging'pathbetween the conductors C321 and C328 of the line 323. Following therestoration of the three relays R520, R540 and R210, the auxiliarytransmitting. and receiving means are ventirely disconnected from theline 323. If desired, the telephone receiver embodied in the substationapparatus 600 may be retained olf its supporting element when the answerswitch III is restored to normal. In such case the substation apparatusis directly connected to the line 323 when the relay R010 restores inresponse to the removal ot ground potential from the start con- -ductorC294, and may be used in the usual mannel'.

As pointed out previously, to initiate a call vat the special substationintended for a substation connected to a line not directly accessible tothe special switching means, such, for example, station B connected tothe line 92|, it is necessary to employ the dial operated calling deviceconventionally included in the substation apparatus 600. Such a call isinitiated in the usual manner by removing -the hand set embodied in theapparatus 600 from its supporting cradle to cause a bridging path to beplaced across the line conductors C321 and C320 of the line 323. As aresult of the completion of this path, one of the non-numerical switchesincluded in the switching equipment 320 responds by extending thecalling loop circuit through to one of the numerical impulse responsiveswitches of the exchange, whereby the seized switch is conditioned torespond to impulses transmitted thereto by way of the calling line 323.When the calling loop circuit is completed, the relays R600 and R690sequentially operate, but without effect since the conductors C294 andC694 are not grounded and accordingly the relay R610 is not energized.The seized impulse responsive switch and one or more additional impulseresponsive switches successively respond to the impulses of successivedigits dialed at the calling substation by extending the connection tothe called line 32| in the usual manner. If, following the dialingoperation, the selected called line is found busy, busy tone is returnedover the calling loop and is reproduced by the receiver included in theapparatus 600 to give an audible indication that the desired connectioncannot be obtained. On the other hand, if the selected called line isidle, ringing current is projected thereover to actuate the signaldevice at the called substation and ringback tone is conducted back overthe calling loop circuit to indicate to the calling party that thecalled substation is being rung. When the call is answered at the calledsubstation, the desired conversational connection is completed in theusual manner.

Following the completion of a connection to the substation B in themanner just described, the auxiliary transmitting and receiving meanscomprising the transmitter microphone |35 and loud speaker |36 may besubstituted for the transmitter and receiver embodied in the hand setfor use in transmitting andvreproducing signal currents.` Suchsubstitution is accomplished by operating the answer switch III to itsoff-normal position to cause the operation of the three relays R210,R520 and R540, and then replacing the hand set of the apparatus 600 uponits supporting element. The three relays noted operate in the exactsequence and perform the same functions as are described in detail in apreceding portion of the specification. In addition, the relay R210,upon operating, completes analternative circuit for energizing the relayR610, this circuit extending from ground at RA212 by way of RA214, C694,RA692RA613 and the lower winding of R610 to battery. is completed therelay R610 operates to complete the holding circuit for R600 and R610,and to perform the various switching functions previously explained.When the hand set of the substation apparatus 600 is replaced upon itssup- When this circuit porting element, the holding circuit for the tworelays R610 and R000 is interrupted, whereby the three relays R000, R690and R610 are caused sequentially to restore in the order named. Therelay R610, upon restoring, connects the line conductors C515 and C516to the line terminals 643 and 644 and opens, at RA612, the pathshort-circuiting the transformer winding 629. Thus, the auxiliarytransmitting and receiving apparatus is operatively connected to theline 323 over a channel including the link line C515, C516, the linkline C296, C291, and the transformer 300.

The purpose of first operating the answer switch Ill and then replacingthe hand set of the apparatus 600 upon its supporting element is toprevent the established automatic switch train from being prematurelyreleased. Thus, if the hand set is replaced upon its supporting elementbefore the answer switch I is operated, the loop circuit extending tothe operated switch train forming a part of the switching equipment 320is interrupted. If the last numerical switch of the train, namely, theconnector switch is of the calling party release type, the establishedswitch train will release during the interval when the calling loopcircuit is open and preceding operation of the answer switch When,however, the answer switch Ill is operated prior to replacement of thehand set upon its supporting element, the relay R210 completes, atRA21I, a multiple loop circuit which precludes the release of theoperated switch train.

It will be appreciated that the regular substation transmitter andreceiver may be substituted at will for the transmitting microphone |35and the loud speaker |36 by removing the hand set or receiver from itssupporting hook or replacing the same thereon, the three relays R610,R690 and R690 cooperating to eiect the transfer in the exact mannerpreviously described.

As indicated in the introductory portion of the specification, a callingparty at the executive substation may utilize the special substationapparatus to initiate a direct call to one of the subordinate linesduring the course of a conversation over an established connection toanother of the lines extending to the exchange housing the automaticswitching equipment 320. If the ensuing conversation is being held overa connection utilizing the regular substation transmitter and receiverembodied in the substation apparatus 600, the second call to thesubordinate line may be initiated in the exact` manner described above.For example, if the calling party is using the regular substationapparatus 600 to converse over an established connection extending tothe substation B and desires to secure information from the subordinatehaving an office in which the substation A is located, the direct callto the substation A is initiated by operating the calling switch to thelower olf-normal position in the manner described above. The codingrelays R200, R2|0 and R220, the common equipment 4|5 and the rotaryswitch 3| 0 respond to the operation of the switch |00 to its off-normalposition by performing the previously described functions, whereby atalking connection including the loud speaker |36 and the transmittermicrophone |35 is established between the executive substation and thesubstation A. As pointed out previously, one of the functions of therelay R450 is that of impressing ground potential upon the amplifierstart conductor C294 thereby to condition the amplifier for operationand to prepare one of the operating circuits for the relay R610. Since,however, the relays R680 and R690 are fully operated when the relay R450operates, this operating circuit for the relay R610 is not completed,`it being held open at the operated armature RA692. Thus, so long as thehand set is not replaced upon its supporting element the transmitter andreceiver thereof are connected to the line 323 for communicationthereover. Also, the auxiliary transmitting and receiving meansareoperatively connected for use in communicating over the establisheddirect line connection. After the two connections have been establishedif the hand set is replaced upon its supporting element and is thenremoved therefrom, the relay R610 is caused to operate to disconnect theauxiliary transmitting and receiving apparatus from the direct lineconnection, to disconnect the substation apparatus from the line 323 andto connect this apparatus for use in communicating over the establisheddirect llinie connection. More particularly, when the hand set isreplaced upon its supporting element, the calling loop circuit isinterrupted causing the two relays R630 and R630 sequentially torestore. 'Ihese relays reoperate sequentially when the hand set isthereafter removed from its supporting element, the relay R600 rstclosing the X contacts associated with RA633 to complete the previouslytraced operating circuit for the relay R610. Thus, the relay R610 iscaused to reoperate to effect the above-described transfer operation.

If, with the two connections established it is desired lto connect thehand set to the direct line connection without releasing the firstconnection, namely, that extending by way of certain of the automaticswitches of the switching equipment 320, the answer switch and thetransfer switch ||0 are sequentially operated to their off-normalpositions. When the answer switch I I i is operated the relays R210 andR520 sequentially operate and the relay R540 restores, all in the mannerpreviously explained. The relay R210 completes, at RA21I, theabove-traced multiple calling loop circuit, thereby to prevent therelease of the operated switches in the equipment 320, and completes, atRA212 and RA214, the above-traced alternative circuit for energizing thelower winding of the relay R610. The relay R610 now operates to completethe previously traced holding circuit for itself and the relay R630.With the three relays R210, R520 and R610 operated the substationapparatus is operatively connected by wayv of the link line C515, C515,the link line C236, C291 and the transformer 300 to the line 323. Whenthe transfer switch 0 is operated following operation of the answerswitch the relay R520 restores to dis- -connect the substation apparatusfrom the line 323 and to connect this apparatus for use inccmmunicatingover the established direct line connection. Obviously, byoperating the transfer switch ||0 between its two positions thesubstation apparatus 600 may be transferred from one connection to theother at will. Also, by removing the hand set ofthe apparatus 600 fromits supporting element and replacing the hand set upon its supporting'element the hand set and the auxiliary transmitting and receiving meansmay be interchangeably connected to either of the two establishedconnections. 'I'he switching apparatus is released in the manner setforth above when the operated calling switch |00, the answer switch andthe transfer switch ||0 are restored to normal.

If the loud speaker |36 and the transmitter |35 are being used as thereceiving and transmitting means in an established connection extendingto the substation B, for example, and

it is desired to use the special substation appa.-l

ratus to initiate a call to one of the subordinate lines, as, forexample, the line 322, directly accessible to the special substationequipment,` it is necessary to operate the calling switch |00 to theoff-normal position corresponding to the line 322 and following this, tooperate the transfer switch l0 to its oil-normal position. When thecalling switch |00 is operated to the oir-normal position correspondingto the line 322, the apparatus illustrated in Figs. 2, `3, 4 and 5operates to cause a connection to be extended to the line 322 in theexact manner previously described. In this regard it will be recalledthat the three relays R210, R520 and R540 are operated. Hence, theamplifier comprising the vacuum tube 620 is conditioned for operationand the line conductors C515 and C516 incoming to the anti-side-toneimpedance network 630 are connectedthrough to the repeating coil 300.The operated condition of the three relays R210, R520 and R540 in no wayinterferes with the operation of the other relays included inthe commoncontrol apparatus 4I5Ato cause the connection to the desired subordinateline 322 to be be established. When the transfer the relay R520 fallsback, it disconnects the line conductors C515 and C516 from across theterminals of the transformer winding and connects these line conductorsto the respective terminals of the transformer winding 50|, therebyoperatively to include the auxiliary transmitting and receiving meanscomprising the loud speaker |36 and the transmitter microphone in theestablished connection extending to the called subordinate line 322. Therelay R520, upon restoringfalso disconnects the conductor C514 from theconductor C513 and connects it to the conductor C512, thus substitutingthe line balancing circuit comprising the inductance element 63| and theresistor 632 for the balancing circuit comprising the inductance element63| and the resistor 632' in the anti-side-tone impedance network 630.With the relay R520 in its restored position, the original circuit forsupplying energizing current to the microphone |35 and for holding R540operated, is interrupted at RA528'. At this time, however, the backbridge relay R5|0 is in its operated position so that a multiple circuitextending from ground at RA5| i' and b-y way 'of RC5|3 and RA532 iscompleted for maintaining R540 operated and for supplying energizingcurrent to the microphone |35.

It will lbe noted that to effect the transfer operation just describedthe transfer switch ||0 is operated to its off-normal position withoutrestoring the answer switch toits normal position. So long as thelast-mentioned switch is' retained in its off-normal position, theabovetraced circuit by way of Vwhich vthe relay R210 is energized isheld completed and. so long as R210 is energized and remains in itsoperated position, the loop circuit extending tothe substation B andincluding the line-322 is maintained completed at RA21I. After the twocon- 75

